CEL on after engine warms up..........

[dpfurby]

I have a 1994 Ford Ranger- V6 4 liter engine that runs good---- but the check engine light comes on after the engine warms up--- on the way to work it comes on in the exact same intersection!!!

It has the EEC IV computer and I read the "blinks" from the CEL.

The readings were-- 167, 177 and 157.

I have replaced with new sensors---

both O2 sensors
mass air flow sensor
intake air charge sensor
crankshaft position sensor
coolant temp. sensor ("electronic choke".)

along with these new parts--
properly gapped plugs
plug wires
coil pack
air filter
fuel filter
PCV valve

All vaacuum hoses checked.

Still the cel light comes on after warmup....

Any suggestions or help appreciated!!!!

 

[Edgefevah]

Try running some injector cleaner through it, a injector might be plugged and causing you to run to lean/rich.


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[SenorNoob]

Have you checked the wiring going to those sensors? Also, a wise person once said: "New doesn't mean good. It just means untested.".

 

[RonD]

167 is a test code so ignore it unless you were doing a "goose" test
"Goose" test is when you press gas pedal down at least 3/4 then release it, you would be testing the Throttle Position Sensor(TPS)

157 Mass Air Flow signal is/was low or grounded
could be wiring or MAF or Computer
MAF sensors just need to be cleaned
Wiring can be checked with ohm/volt meter
Read here
http://www.therangerstation.com/tech_library/CleanMAF.html

Use sewing pins to pierce wires to test voltage at MAF while they are still connected

Computer(PCM) has several grounds that are not shared, these are located around the PCM under the dash, heater core leaks or windshield leaks can cause these to corrode.
Long shot but not "no shot", lol.

177 Oxygen sensor not switching – system was rich Left or Front HO2S
This usually means the O2 sensor IS working, it is reporting rich condition that is not getting better with adjustment.
Could be Fuel Pressure Regulator(FPR) is leaking fuel into the intake via it's vacuum line.
Could be an injector is leaking.
My bet would be if you fix the MAF issue first, the Rich issue will go away.
MAF is the main sensor used for fuel/air adjustment, O2 sensors are just for fine tuning.


The reason you don't get the CEL until it warms up is because most of the sensors are off line, including the O2 sensors, until engine warms up, this is called Open Loop operation, once engine warms up it switches to Closed Loop and uses all the sensors.
On a cold engine the computer runs engine rich with advanced timing and high idle, this is the same as running with a Choke on carb engine.

O2 sensors don't work until they are heated to 650 degF, even after a restart with engine warmed up the O2 sensors are not used for a minute or two to allow them to reheat.
If you have heated O2 sensors make sure to check the power fuse for them.

The only data the computer has on lean/rich is the O2 sensor, once engine warms up that data becomes available, so computer is leaning out Left bank but O2 sensor is still reporting too rich, computer can't shut off injectors entirely, lol, so once it reaches it's lowest lean setting for the injectors it turns on the CEL letting you know there is a rich condition it can not compensate for.
Pull the spark plugs and check if they are getting black tips, a sign of too rich a mix.